In earlier impact tools such as that described in U.S. Pat. No. 3,770,322, issued to Cobb et al. on Nov. 6, 1973 and also in U.S. Pat. No. 3,922,017, issued to Cobb on Nov. 25, 1975, the reciprocated hammer is driven by an eccentric cam to which the impactor is journaled. In the manner described in these patents, energy is constantly utilized to reciprocate the hammer under load or no load conditions. As a result, power requirements are higher than necessary. Additionally, ambient noise from the impacting device is constant under load or no load conditions.
Eccentric impactors of the type described in the above patents generally provide non-linear motion to the impactor. Although the non-linear motion may be converted to linear motion at the hammer by various schemes, some energy is lost during the conversion process. Commonly, it has been found that a ball and socket type joint, similar to that described in U.S. Pat. No. 3,922,017, is satisfactory to convert the non-linear motion of the eccentric to linear motion at the hammer. Other schemes could include a pivoted arm arrangement or the like, such as disclosed in U.S. Pat. No. 3,770,322. In both these earlier schemes, a thrust bearing is required between the eccentric cam and the impactor portion of the device.
In eccentric driven impactor devices of the type described herein, it is usually necessary to seal the rotating eccentric or driving mechanism from the environment in which it operates. This is necessary because of the generally highly abrasive nature of the material being broken. This material usually is rock of some form or another and would certainly have detrimental effects on bearings and attendant components in the drive mechanism. Because of the reciprocating nature of the impactor operating on the hammer, a flexible seal is necessary between the drive chamber and the hammer. Fatigue in the flexible seal becomes a real problem and must be guarded against by both the manufacturer and the user. It is well known that fatigue in any member may be measured by the number of flexures of that member.
It is elementary that useful life may be lengthened by either increasing the available number of flexures in the material or decreasing the absolute number of flexures of the material. Materials suitable for flexible seals of the type envisioned herein are well developed and it is unlikely that a substantial increase in the mean time before failure will occur in such seals in the immediate future. Accordingly, the manufacturer of impacting devices is desirous of decreasing the number of flexures, particularly unnecessary flexures which occur during repositioning of the impactor device, notwithstanding efforts of the operator to shut-down the machine during such times. Further, it is appropriate to limit the degree of flexure by limiting deflection of the impactor to that necessary to accomplish the job.
In some conventional devices, the throw of the impactor is constant irregardless of the work to be accomplished. Control of the force applied to the hammer by the constantly reciprocating impactor is usually accomplished by positioning the hammer at varying distances from the impactor. Therefore, if a relatively light breaking force is appropriate, the hammer or other work member is positioned relatively further away from the impactor so that impact on the hammer for delivery to the workpiece is present only at the extreme throw of the eccentric. Nevertheless, full flexure of the case seal occurs on each blow of the impactor.
In another type of impactor drive mechanism, such as disclosed in U.S. Pat. No. 3,868,145, granted to Cobb et al. on Feb. 25, 1976, the impact member is an annular ring journaled on an eccentric shaft, so that the impact member contacts the hammer upon the orbital rotation of the ring on an eccentric shaft. In this particular type of drive mechanism, the impact ring is rotated either in a regular or an irregular pattern to prevent flattening of the faces of the impactor ring, thus necessitating a replacement or change. Rotation is accomplished in several ways, such as providing conical washers which rotate the impactor ring. Nevertheless, the resulting pattern formed by the ring upon rotation is non-linear. Thus, at the point of contact of the impact ring with the hammer, a "wiping" action may occur across the face of the hammer, resulting in unnecessary wear on the hammer and the impactor.